The aim of this paper is to study ontology-mediated querying on
classes of databases of bounded cliquewidth. This is relevant because
such classes are maximal known ones on which ontology-mediated
querying with DLs such as and is in in data
complexity, even when unions of conjunctive queries (UCQs) are
admitted as queries. This follows from a folklore translation of
ontology-mediated querying into MSO\(_1\)  and the known result that
MSO\(_1\)  has data complexity on structures of bounded
cliquewidth [1]}. In fact, it even
has linear time data complexity (when a \(k\) -expression for
constructing the database is provided). We recall that bounded
treewidth implies bounded cliquewidth since every database of
treewidth \(k\)  has cliquewidth at most \(3 \cdot 2^{k-1}\) 
[2]}. The converse is false because
databases of bounded treewidth are sparse in the sense of
graph theory, whereas databases of bounded cliquewidth may be dense.
